1. Field of the Invention
This invention relates to etching of copper and, more particularly, to new and improved alkaline copper etching working baths and a process for dissolving copper, wherein the etch rate is increased while etching of the side walls of the copper substrate is retarded.
2. Description of the Prior Art
In the manufacture of printed circuit board laminates comprised of a phenolic or epoxy-glass core, metallic copper is typically laminated to one or both sides of the core. A circuit is made by using an etch-resist pattern to render the appropriate parts of the copper on the board resistant to etching so that when the entire board is contacted with an etching bath all the copper except that required for the circuit is dissolved away. The etch-resist pattern is so named because of its resistance to attack by the etching bath.
In the prior art practice, the copper-clad insulating board bearing the etch-resist pattern was contacted with an acidic ferric chloride or an alkaline ammoniacal cupric chloride etching solution. This was done either by immersing the board and the resist affixed thereto in the etching solution or the etching solution was so applied that droplets impinged in principally perpendicular directions against the copper surface and the resist. The acidic ferric chloride or alkaline ammoniacal cupric chloride etching solution attacked the copper where the metal surface was not protected by the resist and, by chemical action, removed the copper. The copper surface protected by the resist was not subject to a direct attack by the etching solution. As the etching proceeded, the resist-covered portions of the copper stood out in vertical relief. As the depth of the etch increased, the sides of the copper supporting the resist were exposed to the etching solution. The acidic ferric chloride or alkaline ammoniacal cupric chloride etching solution then attacked the exposed metal side walls and undercut the resist around its edges. If not skillfully controlled, this undercutting resulted in circuit lines that were not true representations of the etch-resist pattern transferred to the copper; the circuit lines became narrowed, exhibited lower conductivity, and, upon occasion, the edges of the lines broke off and created short circuits.
In general, where the etch rate is increased, undercutting of the side walls correspondingly is increased. This has discouraged accelerated etching of copper circuit boards. Attempts to control undercutting of the side walls while maintaining the etch rate with aqueous alkaline ammoniacal etchants have heretofore been commercially unacceptable.